When supercharging, does the FMU have to be installed after the normal fuel pressure regulator or can it be installed before it? I see most people install the FMU last, before the fuel returns to the tank. I have future plans of running a low boost supercharger and a wet 50 or 75 shot of nitrous. If the FMU could be installed before the normal fuel pressure regulator I could pull the fuel for the nitrous kit from after the FMU and before the regulator. If not, I'll need a separate regulator and return line just for the nitrous kit. Is there any reason the FMU is normally installed after the normal regulator?

 

the fmu increases fuel pressure by restricting the return line size inside of the fmu thus increasing the fuel pressure...........it must be in the return line........just pull fuel from the fuel rail, thats what I did.

 

You could swap places and put the stock reglator after the FMU. Then you will get 'stock' fuel pressure between the FMU and stock regulator.

They must both be mounted in series on the fuel return after the fuel rail. The one that wants the highest pressure wins.

 

But when the FMU raises the fuel pressure substantially, doesn't that screw with the nitrous enrichment fuel? You could jet it accordingly but as the boost pressure goes up it would screw up the air fuel ratio, I think.

 

That is what I was hoping for and thinking of trying. It seems right, was just hoping someone who has tried it could confirm. Have you mounted an FMU after the fuel rail but before the afpr and had it work ok? I was thinking the same as you, the one with the higher pressure "wins".

 

forced induction is not easy..............your playing on the edge using both blower and nos..........even with a low boost and low nos shot....I would recomend one or the other...............maybe a small dry nos shot just to cool the supercharger charge.

 

I know I'll be on the edge. I'm spraying a 175 shot now, with stock bottom end, HSR intake, CC503 cam, reworked 113 heads, headers, and many many other mods. 10.0:1 SCR. It is surviving good so far. On a Land and Sea dyno (similar to a Mustang dyno) I made roughly 300 rwhp without spraying. I am thinking a P600B kit, with intercooler, run at 6-8 psi, should give me roughly 400 rwhp and I think the engine should survive as it is surviving now. Would like to try a 50 shot then, and if the bottom end gives out then I'll go to a forged 383. That is my plan as of now, not all happening overnight but....

Anyway, what difference does it make if I spray a wet or dry shot? A wet shot will cool the intake charge just the same. Just have to spray fuel in also, like I do now. I have a Zex wet kit on it now. Do I have to spray a dry shot?
Using the nitrous to cool the charge is what is so impressive on a supercharged engine. I am planning on running an intercooler and also running 1 degree of timing retard per pound of boost (with a timing retard box). Should be safe there, will datalog to make sure. With a little more timing retard and a 40 or 50 shot it should make some serious power. If I keep air fuel ratio and the timing good and safe it may even survive for a year on the stock bottom end.

 

Both the FMU and a stock regulator works exactly the same. They increase the fuel pressure by restricting the return flow. They just have a different pressure ratio. The one that wants to restrict the flow most will set the pressure.

What I said before is not true however. I thought about this again.

If the FMU wants 50 psi and the stock regulator 30, then the FMU will win and both see a reference pressure of 50 psi. As the stock regulator sees 50 psi it will fully open. The pressure differance over the stock regulator will be almost nothing and almost all 50 psi pressure drop will be over the FMU.

 

So, if I mount the FMU before the "stock" regulator (I have an aftermarket in the return line), when the FMU calls for more than 45 psi it will take over. The pressure between the FMU and the "stock" regulator would be whatever the "stock" regulator is set at, which would be about 45 psi. The only thing I would lose is the 1:1 boost reference in the stock regulator because the FMU would already be at a higher pressure as it is rising at say 7:1. I could just set the FMU to 8:1 to make up for it, right? It seems like it could work.

Also, I'm assuming that an FMU can't be set to 45 psi when vacuum or no boost is present, otherwise you wouldn't need a "stock" regulator at all, you could just set the FMU to 45 psi. I'm assuming that the FMU just takes whatever pressure it is seeing and begins raising it on a ratio when boost starts. Is that correct? I don't know much about FMU's and am learning here.

 

There has been a lot of discussion of which regulator will take precedence (FMU or AFPR). The one that requests the higher psi (which will be the FMU when WOT under boost) will take precedence no matter whether it is mounted upstream or downstream of the factory regulator.

However, if you mounted the FMU first, then fed the N2O from the feed between the FMU and the latter mounted factory AFPR, then you would have a lower psi feed for the N2O system. But, I would be concerned about getting a consistant feed of fuel to the N2O system with an FMU that is "hi-jacking" the fuel away from the N2O feed at rates of 50-85 psi in mere fractions of a second. I would not go this route.

I ran a seperate system (actually most of the factory system) to feed just the N2O system in my car. I used a modded factory TPI regulator and fed off one side to a -6AN to feed the fuel solenoid and returned from the modded factory regulator through the original return line. The modded AFPR has the vacuum line capped for a consistant psi at all times. The engine fuel supply is a completely separate system feeding from the Weldon pump through an aftermarket fuel system.
Aaron

 

Thank you for the answers! I was worried about the nitrous feed not getting any fuel for an instant or 2, I was considering not spraying till after the boost was in quite a bit till I knew the FMU was returning some fuel and not holding it all back. If it won't work then I'd be better to get bigger injectors and a new tune then to run a whole separate fuel system with the way my car is now.

Another possible option, is there a type of regulator or reducer that lets a certain psi through no more, no return line needed? Maybe I could pull nitrous fuel from the fuel rail, then use this device in the feed line to the nitrous fuel solenoid. That way, when the FMU raises the fuel pressure, this device would only let a set psi (say 45 psi) through it. Anyone ever heard of such a device?

 

No ... Read the post where I corrected myself again.

When the FMU is in control the fuel pressure before the stock regulator could drop to nothing when mounted after the FMU.

If the FMU is a 8:1 unit and it is set to 45 psi at atmospheric reference pressure and the stock regulator 1:1 is set to 45 psi, this is what you get:
At 1 psi boost the stock regulator will se a too high reference pressure. It will try to lower the pressure by reducing the restriction. The pressure drop over the regulator will be way less than 45 psi. It is not possible to use that as a fuel source for nitrous fuel.